The Moon will reach the
closest point along its orbit to the Earth and will appear slightly larger than
at other times.

The Moon's distance from the Earth varies because its orbit is not perfectly
circular – it is instead slightly oval-shaped, tracing out a path called
an ellipse.

As the Moon traverses this elliptical path around the Earth each month, its
distance varies by around 10%, between 363,000 km and 405,000 km.
Its angular size also varies by the same factor, and its brightness also
changes, though this is hard to detect in practice, given the Moon's phases are
changing at the same time.

The exact period of the Moon's cycle between perigee (closest approach), apogee
(furthest recess) and back again is 27.555 days – a period of time
called an anomalistic month. This is very close to the Moon's orbital
period (27.322 days), but slightly longer. For more information on why these
periods don't exactly match, see In-The-Sky.org's glossary article for the term
month.

As the perigee of 13 September 2017 will occur close to the time of new moon, the moon will appear as no more than a thin crescent.

On this occasion the Moon will pass within a distance of
369,000 km of the Earth, and appear with an angular diameter of
32.29 arcsec. This may be compared to its average size of 31.07 arcmin.

The Moon illusion

The Moon's cycle between perigee and apogee is a genuine variation in the
Moon's angular size. This should not be confused with the Moon illusion
– an optical illusion that makes the Moon appear much larger than it
really is when it is close to the horizon. The reason why we experience this
optical illusion is still hotly debated.

Source

The circumstances of this event were computed using the DE405 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).

This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.